It is known in the art to form a thin paper web from a slurry of water and fiber, dewater the wet web, and then at least partially dry the dewatered web. In the manufacture of tissue and similar paper products, creping is commonly used on such dewatered webs to impart desirable properties, such as softness and bulk. Creping is typically accomplished by conveying or carrying the web on a fabric to a heated rotary drum termed in the art a Yankee dryer. The web commonly is transferred to an adhesive dryer surface of the dryer and carried around a major circumferential portion of the dryer before the web reaches a zone of web de-contact from the drum. The de-contact zone is equipped with a creping blade against which the web abuts so as to be pushed backwardly or compacted upon itself in a machine direction of the web and attain the well-known tissue crepe paper structure, at which point the resulting creped web is removed from the dryer and collected, usually in rolled up form.
Before the web is transferred to the Yankee dryer, typically an adhesive composition, sometimes referred to as a “coating package” in the industry, is applied directly to the dryer surface of the dryer to form the adhesive dryer surface. The creping action typically requires some adhesion of the web to the outer surface of the dryer to effect a consistent and uniform creping action. Creping adhesives alone or in combination with release agents or other adjuvants have been applied either to the web or to the surface of the dryer in efforts to provide some balance of adhesion and release between the web and the dryer surface for purposes of drying and creping.
Various properties of the creping adhesive can be factors in the creping performance obtained. The rewettability of the creping adhesive on the dryer surface can be one such factor. An adhesive which can rewet on the surface of the dryer may improve retention of the web on the dryer surface through creping and assist in reducing buildup on the drum and on the creping blade. Many conventional creping adhesives are not rewettable. Further, coating buildup can appear as a build-up of adhesive on the rear surface of the creping blade, such as along the edges or corners of the creping blade. This adhesive build up can cause chattering or bouncing of the blade. Eventually, portions of the web may skip underneath the creping blade, causing picks or holes in the removed creped web, which may lead to web breaks and machine downtime. The level of adhesion of the creping adhesive to the drum dryer surface can be another factor which affects creping performance and results. Inadequate adhesion can result in poor creping, sheet floating, poor sheet handling, or other problems, whereas excessive adhesion may result in crepe blade picking, web plugging behind the crepe blade, web breaks due to excessive tension, or other problems.
Various types of creping adhesives have been used to adhere fibrous webs to rotary dryers such as Yankee dryers. Creping adhesives have included, for example, polyvinyl acetate-ethylene copolymer emulsions and aqueous polyvinyl alcohol solutions. It has been found that conventionally used polyvinyl acetate-ethylene copolymer compositions, which may contain small percentages of polyvinyl alcohol such as less than about 5% of the total solids by weight, may be generally adequate for the purpose but can cause a number of undesirable effects, such as blocking problems and others as mentioned in U.S. Pat. No. 6,991,707 B2, which is incorporated herein by reference in its entirety. Polyvinyl alcohol compositions (which may contain some polyvinyl acetate) can pose similar problems when used as creping adhesives, and can tend to coat the dryer with a hard and uneven film that builds up as drying and creping proceed, resulting in uneven creping or other problems.
Other creping adhesives have included wet strength resins, such as polyamidoamines cross-linked with epihalohydrin (PAE). PAE resins are described, for example, in U.S. Pat. Nos. 2,926,116; 7,943,705 B2; and 7,718,035 B2. PAE resins are generally prepared by reacting an epichlorohydrin and a polyamide containing secondary or tertiary amine groups, followed by stabilizing the reaction products by acidification with sulfuric or hydrochloric acid. The creping adhesive desirably should be “rewettable,” which is not a property of many conventional PAE resins as previously synthesized and used. A non-rewettable adhesive can result in buildup of adhesive on the dryer surface or cause other problems.
The present investigators have determined that creping performance and product quality in the manufacture of creped paper products can be enhanced by increasing the pH of settable or curable coating packages before application to a dryer surface.